Generally, exhaust gas flowing out from an engine through an exhaust manifold is driven into a catalytic converter mounted on an exhaust pipe and is purified therein. After this purification, the noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through the tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matter (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification catalyst (DeNOx catalyst) is one type of catalytic converter, and it purifies nitrogen oxide (NOx) contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbon (HC) are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced in the DeNOx catalyst through oxidation-reduction reaction with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst is used as such a DeNOx catalyst. The LNT catalyst absorbs the NOx contained in the exhaust gas when the air/fuel ratio is lean, and releases the absorbed NOx and reduces the released nitrogen oxide and the nitrogen oxide contained in the exhaust gas when the air/fuel ratio is rich.
If the temperature of the exhaust gas, however, is high (e.g., the temperature of the exhaust gas is higher than 400° C.), the LNT catalyst cannot purify the nitrogen oxide contained in the exhaust gas. Particularly, if a vehicle runs under high speed or high load conditions, the temperature of the exhaust gas will be high and the LNT catalyst disposed closely to the engine cannot absorb the nitrogen oxide in the exhaust gas. Therefore, the nitrogen oxide in the exhaust gas may be discharged from the LNT catalyst. In addition, since the flow of the exhaust gas is greater under high acceleration or high load conditions, NOx exhausted to the exterior of the vehicle may be increased. Therefore, the purifying performance of the nitrogen oxide may be greatly deteriorated.
In a catalyzed particulate filter (CPF), a catalyst is coated in the particulate filter to enhance a function of removing the particulate matter or additionally removing the pollutants.
In the CPF, the catalyst is coated on the porous wall that separates the inlet channel and the outlet channel from each other, and the fluid passes through the porous wall and comes into contact with the catalyst coating. There is a pressure difference between the inlet and outlet channels separated by the porous wall. This allows the fluid to pass quickly through the porous wall. Accordingly, the contact time between the catalyst and the fluid is short, which makes it difficult for the catalytic reaction to occur efficiently.
Furthermore, a thick catalyst coating on the porous wall may cause the catalyst to block the micropores on the wall, and this may disturb the flow of the fluid from the inlet channel to the outlet channel. Accordingly, the back pressure increases. To minimize the increase in back pressure, a catalyst is thinly coated on the walls in the CPF. Thus, the amount of catalyst coating on the CPF may be insufficient for the catalytic reaction to occur efficiently.
To overcome this problem, the surface area of the walls to be coated with the catalyst may be increased by increasing the number (density) of inlet channels and outlet channels (hereinafter, collectively referred to as ‘cells’). However, the increase in cell density in the limited space reduces the wall thickness. The reduction in wall thickness may deteriorate the filter performance. Therefore, the cell density should not be increased to more than the density limit.